JP2008516883A - Method for crystallizing soluble salt of divalent anion from salt water - Google Patents

Abstract

The invention pertains to removing soluble alkali metal or ammonium salt of a divalent anion from brine comprising following steps: obtaining brine with NaCl-concentration between 150g/L and saturation in the presence or absense of a cyrstal growth inhibitor for NaCl(GCI-NaCl),or with NaCl concentration above saturation in the presence of a CGI-NaCl, said brine optionally comprising a crystal growth inhibitor for the alkali metal or ammonium salt of the divalent anion(CGI-DA); if necessary, acidify the solution to pH<11.5; if the concentration of CGI-DA is less than 20 mg/L, adding CGI-DA to obtain at least 20 mg CGI-DA/L; subjecting the solution to a membrane filtration; if the concentration of CGI-DA in the concentration from the separation is less than 20 mg/L, adding CGI-DA to obtain at least 20 mg CGI-DA/L; crystalling the concentration; removing the crystallized alkali metal or ammonium salt of the divalent anion.

Description

  The present invention provides a soluble alkali metal or ammonium salt of a divalent anion by crystallizing these salts from a concentrated aqueous solution containing an alkali metal or ammonium ion and a divalent anion from the salt water. It relates to a method for removing at least part.

Brine is typically produced by dissolving the natural source NaCl in water. Therefore, brine usually also contains divalent anion impurities, typically SO ₄ ²⁻ , CO ₃ ²⁻ and monovalent cation impurities such as K, Li, and / or NH ₄ ⁺ . Most of these anions and cations should be removed in the production of solid sodium chloride, generally by crystallization by evaporation of brine, and in the production of products in which brine is used as a raw material, for example in the production of chlorine. is there. In the method of crystallization of sodium chloride by evaporation, for example, the resulting salt has defects in the crystal lattice and contains occlusion, ie, a small pocket of mother liquor of the crystallization process by evaporation (present in the cavities in the salt crystals. ). Because of these deficiencies and occlusions, sodium chloride and the brine produced therefrom are contaminated with compounds present in the mother liquor. In particular, the amount of SO ₄ ²⁻ and / or CO ₃ ²⁻ that will eventually be in the sodium chloride is a problem in many applications of the sodium chloride produced. Traditionally, additional washing and drying steps, such as energy consuming centrifugation steps, have been used to reduce the concentration of contaminants. The contaminants need to be removed, especially if salt water made from salt or wet salt is used in modern membrane electrolysis cells. For these reasons, methods have been developed to (partially) remove divalent anionic impurities, particularly sulfate, from brine. For example, European Patent Application 0 821 615, for example, positively applies saline containing more than 50 g / l sodium chloride to selectively reduce the concentration of sodium chloride relative to the concentration of sodium sulfate or sodium dichromate in the brine. Disclosed is a nanofiltration method for filtering saline containing, for example, sodium sulfate or sodium dichromate, by supplying the nanofiltration membrane module under a given pressure to provide a concentrate and permeate. However, sulfates or dichromates can only be removed up to their solubility limit. Thus, brine containing high concentrations of these salts must be removed, which is undesirable from both an environmental and economic point of view. This is because it leads to significant salt loss.

US 6,036,867 is a method for desalting and demineralizing aqueous solutions containing acid and / or metal salts, wherein the contaminating salts are removed by crystallization from a solution in which the salt is supersaturated. A method is disclosed. This method includes the following steps:
-Introducing an inhibitor for inhibiting precipitation of a given salt in the solution to be treated;
-Concentrating the salt to a supersaturated concentrate, for example by subjecting the solution to nanofiltration,
-Removing the precipitation inhibitor, and then
-Precipitating crystallizable supersaturated salts, such as calcium sulfate, in the concentrate.

  It is disclosed that this removal of the inhibitory effect of the precipitation inhibitor is necessary in order to be able to carry out precipitation / crystallization of supersaturated salts.

  No. 6,036,867 is not concerned with sodium chloride solutions having a sodium chloride concentration of at least 150 g / L and this reference crystallizes soluble alkali metal or ammonium salts in divalent anion brine. There is no disclosure. In addition, when such methods are used for crystallization of salts that are readily soluble in brine, such as ammonium or alkali metal salts of divalent anions, conventional separation techniques can be used for primary nucleation. It has been found that a slurry is formed containing very small crystals and crystal agglomerates that can hardly be used and separated from the mother liquor. Therefore, washing the salt slurry and separating the salt slurry from the mother liquor in such a way that typically the required wet content of the slurry is below 10% by weight is a very expensive process.

  In view of the above, in one way, not only does it produce a saline solution that is at least partially free of contaminants containing divalent anions and is therefore suitable for further processing, but at the same time divalent anions. Improved method for removing readily soluble alkali metal or ammonium salts of divalent anions from concentrated salt aqueous solutions such that salts containing can be separated such that they are available for further use There is a need for.

  Surprisingly, for the alkali metal or ammonium salts of divalent anions, using membrane filtration and subsequent crystallization of the salt from the resulting concentrate in the presence of at least 20 ppm crystal growth inhibitor. After further concentration of the aqueous salt solution in the presence of a crystal growth inhibitor, it contains an easily soluble salt of a divalent anion, such as an ammonium or alkali metal salt, concentrated alkali metal or ammonium ion and a divalent anion. It has now been found that it can be separated from an aqueous salt solution. Unexpectedly, it has been found that it is possible to reduce supersaturation for the salt, i.e. to crystallize the salt, even in the presence of a crystal growth inhibitor, even if a relatively large amount of at least 20 ppm is used. did. The presence of a crystal growth inhibitor during the crystallization process has the effect of preventing primary nucleation of salt crystals, which ultimately results in a large average particle size, preferably above 500 μm, most preferably above 1 mm. It was observed to result in the formation of relatively coarse salts (ie crystals having a diameter of about 300 microns) with a narrow particle size distribution with a low impurity concentration and a low impurity concentration. These crystals were found to be easily separable from the aqueous slurry, for example by filtration. The narrow grain size distribution also makes it possible to apply a conventional centrifuge.

More particularly, the present invention thus provides a solution of divalent anions from aqueous salt solutions containing aqueous alkali metal or ammonium ions and divalent anions, including crystal growth inhibitors for alkali metal or ammonium salts of divalent anions. An improved method for removing at least part of a soluble alkali metal or ammonium salt comprising the following steps:
-Having a sodium chloride concentration between 150 g / L and saturation in the presence or absence of a crystal growth inhibitor for sodium chloride, or above saturation in the presence of a crystal growth inhibitor for sodium chloride Obtaining an aqueous salt solution having a sodium chloride concentration of: said aqueous salt solution optionally comprising a crystal growth inhibitor for an alkali metal or ammonium salt of a divalent anion;
-Acidifying the solution to a pH below 11.5, if necessary, while maintaining a sodium chloride concentration of at least 150 g / L;
-If the concentration of the crystal growth inhibitor for the alkali metal or ammonium salt of the divalent anion in the aqueous salt solution is less than 20 mg / L, the resulting aqueous salt solution should contain at least 20 mg / L of the alkali metal or ammonium salt of the divalent anion. Adding an amount of the crystal growth inhibitor to include a crystal growth inhibitor for
-The resulting solution is subjected to a membrane filtration step, whereby the aqueous salt solution is subsaturated for a salt water stream (concentrate) that is supersaturated for salts containing divalent anions and for salts containing divalent anions. Separating it into a salt water stream (permeate);
-If the concentration of the crystal growth inhibitor for the alkali metal or ammonium salt of the divalent anion in the concentrate is less than 20 mg / L, the concentrate will contain at least 20 mg / L of the alkali metal or ammonium salt of the divalent anion Adding an amount of the crystal growth inhibitor to include a crystal growth inhibitor for
-Subjecting the resulting concentrate to a crystallization process;
-Removing the crystallized divalent anion alkali metal or ammonium salt; and
-Optionally relates to a method comprising the step of recycling at least a portion of the mother liquor of the crystallizer to an aqueous salt solution for re-use in the membrane filtration step

  A particular benefit of the method of the present invention is that no or little liquid removal is required compared to conventional processes since the alkali metal or ammonium salt of the divalent anion is separated from the concentrate by crystallization. That is. Therefore, significantly less waste is produced. Moreover, using the method of the present invention, two valuable products can be obtained in a single process: purified brine and a salt containing a divalent anion.

  “In the presence or absence of a crystal growth inhibitor for sodium chloride, having a sodium chloride concentration between 150 g / L and saturation, or above saturation in the presence of a crystal growth inhibitor for sodium chloride The expression “obtaining a salt solution having a sodium chloride concentration of” gives a salt solution having a sodium chloride concentration of at least 150 g / L, and the salt solution has a sodium chloride concentration between 150 g / L and saturation. A crystal growth inhibitor for sodium chloride can optionally be present, but if the aqueous salt solution has a sodium chloride concentration above the theoretical saturation concentration, it suppresses crystal growth for an effective amount of sodium chloride. It means that the presence of an agent is required to prevent sodium chloride from precipitating during the membrane filtration process and consequently prevent clogging of the membrane. The term “effective amount” means that the crystal growth inhibitor is added in such an amount that it can prevent primary nucleation of sodium chloride and thus prevent precipitation of sodium chloride during membrane filtration. means.

  The use of an aqueous salt solution containing such a high sodium chloride content is desirable and convenient. This is because it obviates the cumbersome dilution procedure of brine that would otherwise be required before the filtration step and the post-concentration concentration step. Moreover, these high sodium chloride concentrations are actually necessary in order to be able to carry out supersaturation in the salt concentrates containing the soluble divalent anions of the present invention. Preferably the sodium chloride concentration is at least 200 g / L, more preferably at least 275 g / L, most preferably a saturated sodium chloride solution.

  The aqueous salt solution can be diluted with water if necessary to obtain a sodium chloride concentration between 150 g / L and saturation.

The aqueous salt solution is necessary, i.e. if the aqueous salt solution has a pH value above 11.5, or the aqueous salt solution already has a pH value below 11.5, but the membrane filtration step is carried out at a lower pH value. If should be acidified. Preferably, the aqueous salt solution obtained in the first step of the present invention has a pH value above 11.5, which is then acidified to a pH below 11.5. Acidification is preferably performed with H ₂ SO ₄ or HCl, more preferably with CO ₂ , optionally with other acids. The use of CO ₂ is particularly preferred. This is because OH ⁻ is then converted to CO ₃ ^2- , which is much more efficiently retained during membrane filtration than OH ⁻ . Furthermore, in many cases, OH ^- conversion of CO ₃ to ² in the positive influence on the lifetime and / or stability of the membrane. The brine is preferably acidified to obtain an aqueous salt solution having a pH of 2 to 11.5, more preferably 7 to 11.5, and most preferably 9 to 10.5.

The aqueous salt solution containing an aqueous alkali metal or ammonium ion and a divalent anion that is subjected to the membrane filtration step in the method of the present invention preferably contains very low amounts of Ca ²⁺ and Mg ²⁺ . The Ca ²⁺ and Mg ²⁺ contents are preferably less than 1 mmol / L and less than 0.1 mmol / L, respectively.

  Any conventionally known crystal growth inhibitor for sodium chloride can be used in the process of the present invention. It is noted that the crystal growth inhibitor can also be a mixture of crystal growth inhibitors for two or more sodium chlorides. Suitable crystal growth inhibitors for sodium chloride are preferably humic acid, polymaleic acid, polyacrylic acid, sugar, oligopeptide, polypeptide and more than one carboxylic acid group or carboxyalkyl group and optionally further Selected from the group consisting of polymers having phosphate, phosphonate, phosphino, sulfate and / or sulfonate groups, such as carboxymethylcellulose having phosphate groups. Most preferably, the crystal growth inhibitor for sodium chloride is selected from the group consisting of humic acid, polymaleic acid and polyacrylic acid.

  The divalent anion of the present invention contained in the brine, preferably sulfate or carbonate, has an alkali metal or ammonium counter ion. The counter ion is preferably selected from the group consisting of sodium, potassium, lithium and ammonium. Most preferred is sodium. The salt to be crystallized according to the invention is preferably sodium sulfate or sodium carbonate. Most preferred is sodium sulfate.

  The crystal growth inhibitor for alkali metal or ammonium salts of divalent anions of the present invention can prevent primary nucleation of the salt to be crystallized, allowing operation of the membrane filtration unit without the formation of solids. Can be any additive. It is noted that this crystal growth inhibitor can also consist of a mixture of two or more crystal growth inhibitors. Preferred crystal growth inhibitors for alkali metal or ammonium salts of divalent anions are from the group consisting of polymerates, polyphosphine carboxylic acids (eg Belsperse ™), polyphosphates, polycarboxylic acids, polyacrylic acids and humic acids. One or more selected compounds. Other crystal growth inhibitors can also be used. The crystal growth inhibitor is used in a total amount of at least 20 mg / L, preferably at least 25 mg / L, more preferably at least 50 mg / L, most preferably at least 75 mg / L. For the crystal growth inhibitor, an amount of preferably up to 250 mg / L, more preferably up to 150 mg / L is used.

  Further, to carry out the crystallization of the alkali metal or ammonium salt (eg sodium sulfate) of the divalent anion of the present invention, preferably using the salt species and / or high shear containing the divalent anion. Induces secondary nucleation, thereby providing an additional surface for crystal growth. Usually, a fluidized bed crystallizer is used to obtain a monodisperse product. Other crystallization methods can also be used, but the product quality may be reduced.

As referred to herein, a “membrane” placed inside a membrane filtration unit for separating divalent anions from aqueous salt solutions is designed to selectively reject divalent and other multivalent anions. Any conventional designed and having a molecular weight cleavage of at least 100 Da, preferably at least 150 Da (molecular weight cleavage is up to 25,000 Da, preferably up to 10,000 Da, more preferably up to 2,500 Da, most preferably up to 1,000 Da) It is further noted that it is meant to indicate a membrane, preferably a nanofiltration membrane. The nanofiltration system is preferably a nanofiltration type semi-permeable membrane such as Film Tec ™ NF270 (The Dow Chemical Company), DESAL ™ 5DK, DESAL ™ 5DL and DESAL ™ ) 5HL (all GE / Osmonics), NTR ™ 7250 (Nitto Denko Industrial Membranes) and AFC ™ -30 (PCI Membrane Systems LTD) Use what is sold as)). These membranes and similar membranes suitable for use in the method of the present invention are less than 80%, preferably more than 70%, during processing of a 1 g / L NaCl solution in deionized water in full recirculation operation. As shown by the lower chloride retention, 1 g / L MgSO _{4 in} deionized water in full recycle operation while allowing a high proportion of all monovalent anions, especially chloride and bromide, to pass through the membrane. It is effective to reject a high proportion of all divalent anions, especially sulfate and carbonate, as shown by the observed sulfate retention above 80%, preferably above 90% during solution processing. Nanofiltration-type semi-permeable membranes, such as those listed above, are preferred, but other nanofiltration membranes with these high divalent ion rejection properties are commercially available and can be used alternatively can do.

  By using a spiral wound nanofiltration module with a feed channel-providing spacer material having a thickness of at least about 1 mm, the treated brine is effectively pumped to a pressure of about 1.5 to about 10 MPa, after which It can be applied to the module. At this pressure, it preferably has an axial speed of at least about 10-15 cm per second.

  “Supersaturated for salts containing divalent anions” and “Unsaturated for salts containing divalent anions” (“salts containing divalent anions” are also referred to herein as alkali metal or ammonium salts of divalent anions. The term) is related to a solution without a crystal growth inhibitor for the salt containing the divalent anion, wherein the concentration of the alkali metal or ammonium salt of the divalent anion is divalent from the aqueous salt solution, respectively. Heat of the alkali metal or ammonium salt of the divalent anion, measured at the same temperature and pressure at which the method for removing at least part of the alkali metal or ammonium salt of the anion should be carried out Higher and lower than the theoretical maximum concentration at mechanical equilibrium.

A typical flow chart for removing sulfate (or another divalent anion) from brine is shown schematically in FIG. The salt water (B) is continuously supplied to the salt water buffer tank 1. Alternatively, a batch of brine (eg 10,000 liters) is treated. If necessary, the brine stream is acidified in the scrubber 2 to a pH preferably between about 7 and about 11.5, more preferably about 10.5. If CO ₂ is used for acidification, it can be introduced together with the flue gas, for example through conduit 3. There are also suitable crystal growth inhibitors for alkali metal or ammonium salts of divalent anions to be removed at least in part from the brine.

  The treated brine is flowed through an outlet line 4, which may include valves, pumps, etc., for example by gravity. The outlet line 4 is connected to the inlet of the nanofiltration unit 5. This unit includes several spiral wound nanofiltration modules placed in several equal pressure vessels. These pressure vessels can be operated in more than one stage, optionally recirculating a portion of the concentrate from each stage to the inlet of that stage. The other part of the concentrate is sent to the next stage.

  Although less preferred, water can be added to the feed of the membrane filtration unit to prevent crystallization of sodium chloride inside the membrane filtration unit.

  The stream that is subsaturated for the salt containing divalent anions (permeate) is directed through the outlet conduit 6 whereas the stream that is supersaturated for the salt containing divalent anions (concentrate, sometimes also retentate). (referred to as retentate) is led to the crystallizer 8 through the outlet conduit 7. Each of these conduits can include valves, pumps, and the like.

  The crystallizer 8 can be a fluid bed crystallizer type, a stirred vessel type or any other conventionally used crystallizer. The crystallizer has an outlet 9 for feeding the mother liquor to the conduit 3 or the inlet 4 of the filtration unit 5 and an outlet 10 for collecting the crystallized sulfate.

  In a preferred embodiment, unpurified surface water is used in a solution mining process to produce brine, which is further treated in a brine purification process. Sodium chloride is produced in the evaporation plant and the mother liquor is fed to the process of the present invention shown in FIG. Since surface water is not purified, organic compounds containing multi-carboxyl groups that may be present, such as humic acid, will eventually go to the mother liquor, where they grow crystals for alkali metal or ammonium salts of divalent anions. Act as an inhibitor. Therefore, it is not always necessary to add additional crystal growth inhibitors to the mother liquor. It is noted that so-called unpurified organisms in surface water can be deliberately introduced by adding nutrients to the surface water, resulting in biological activity. If the cellular material thus produced is successfully supplied to the saline, the cells are broken and a natural crystal growth inhibitor is introduced into the saline.

  The method of the invention can also be applied for the purification of recycled brine in electrolysis plants. In order to avoid an unacceptably accumulated amount of sulfate in the recirculating brine, it is necessary to eliminate a certain volume of recirculating brine in the conventional process. However, because of the membrane filtration process of the present invention combined with sodium sulfate crystallization and the use of crystal growth inhibitors for sodium sulfate, this exclusion volume can be kept to a minimum.

The alkali metal or ammonium salts of divalent anions isolated according to the present invention can be used in a variety of different applications known to those skilled in the art. Sodium sulfate can be used, for example, in laundry detergents, wood pulp making and the glass industry. Sodium carbonate can be used in glass production, chemicals such as sodium silicate and sodium phosphate production, pulp and paper industry, detergent production and for water treatment. Potassium sulfate can be used, for example, for the production of acidic sulfate or bisulfate, KHSO ₄ . Potassium carbonate can be used in common applications for producing other potassium compounds.

  Permeates containing only small amounts of divalent anions, such as sulfate and carbonate, can be used as raw materials in the production of sodium chloride or soda.

  The following examples illustrate the invention. Although the present invention has been described with reference to certain preferred embodiments that constitute the best mode presently known to the inventors, various modifications and changes as will be apparent to those skilled in the art are set forth in the appended claims. It should be understood that this can be done without departing from the scope of the claimed invention. For example, a spiral wound membrane module is preferred, but other nanofiltration-type semi-permeable membrane separation devices suitable for treating brine can alternatively be used.

Example 1
Two film types, flat sheet Film Tec ™ NF270 polyamide thin film NF membrane (from The Dow Chemical Company) and flat sheet Desal ™ 5DK polyamide NF membrane (GE / Osmonics) (From GE / Osmonics) was used to perform the experiment. The membrane type was tested simultaneously on a DSS lab stack unit, which was operated in a leaching mode with continuous feed and an orthogonal flow rate of 600 L / hr. A total membrane surface area of 0.144 m ² was installed. A mother liquor sample obtained from a sodium chloride crystallizer at a saltwater production plant in Delfzijl (Netherlands) was fed to the unit. The pH of the mother liquor was lowered to pH 10.4 using concentrated H ₂ SO ₄ solution. In the mother liquor, 90 ppm of humic acid was present. The resulting mother liquor sent to the DSS unit contained in particular 1,140 meq / L SO ₄ ^2- . During membrane filtration at 35 bar pressure and 23 ° C., a concentrate containing about 1,770 meq / L SO ₄ ²⁻ was produced. Most of the concentrate is recycled to the membrane feed line (cross flow operation), while a portion of the concentrate is removed, with a concentration rate of about 1.6 (ratio of new feed stream to removed concentrate stream). ) The membrane showed sulfate retention greater than 90%. Despite supersaturation of sodium sulfate in the concentrate, no crystallization was observed during nanofiltration.

Example 2
Two film types, Flat Sheet Film Tec ™ NF270 Polyamide Thin Film NF Membrane (from The Dow Chemical Company) and Flat Sheet Desal ™ 5DK Polyamide NF Membrane (GE / Osmonics) (From GE / Osmonics) was used to perform another experiment. . The membrane type was tested simultaneously on a DSS lab stack unit, which was operated in a leaching mode with continuous feed and an orthogonal flow rate of 600 L / hr. A total membrane surface area of 0.144 m ² was installed. A mother liquor containing 90 ppm of humic acid obtained from a sodium chloride crystallizer at a saltwater production plant in Delfzijl (Netherlands) was fed to the unit. The pH of the mother liquor was lowered to pH 10.4 using concentrated H ₂ SO ₄ solution. In addition, 10 ppm of Belsperse ™ 164 was added to the mother liquor using pure Belsperse ™ 164. The resulting mother liquor sent to the DSS unit was in particular 1,145 meq / L SO ₄ ^2- , 140 g / L Na ⁺ , 11 g / L K ⁺ , 168 g / L Cl ⁻ and 650 mg / L. Br ^- it contained. During membrane filtration at 50 bar pressure and 29 ° C., a concentrate containing about 1,580 meq / L SO ₄ ²⁻ was produced. Most of the concentrate was recycled to the membrane feed line (cross flow operation), while a portion of the concentrate was removed to obtain a concentration factor of about 1.4. The membrane exhibited sulfate retention greater than 95% and chloride and bromide retention of about -16% and -34%, respectively. Despite supersaturation of sodium sulfate in the concentrate, no crystallization was observed during nanofiltration.

Example 3
For sodium sulfate, a brine containing 10 ppm Belsperse 164 and 90 ppm humic acid as a crystal growth inhibitor is fed to a nanofiltration unit as a feed brine containing 1,140 meq / L SO ₄ ^2- Supersaturated. The feed brine was concentrated to yield a concentrate containing 1,680 meq / L SO ₄ ^2- . The nanofiltration unit was operated in continuous (feed and leaching) cross-flow operation, ie by adding an amount of fresh feed equal to the permeate and concentrate removed from the nanofiltration unit. The experiments consisted of a flat sheet Film Tec ™ NF270 polyamide thin film NF membrane (from The Dow Chemical Company) and a flat sheet Desal ™ 5DK polyamide NF membrane (GE / Osmonics (GE / From Osmonics). The membrane sheets were tested simultaneously in a DSS lab stack unit (operated at an orthogonal flow rate of 600 L / hour). A total membrane surface area of 0.144 m ² was installed. Membrane filtration was performed at an operating pressure of 50 bar and a temperature of 29 ° C. The concentration rate was 1.5 (ratio of the mass flow rate of the new feed to the mass flow rate of the concentrate leaving the nanofiltration unit). During the concentration process, no crystallization of sulfate was observed.
It was then investigated whether the supersaturation could be reduced by the simple addition of crystals of solid sodium sulfate (Na ₂ SO ₄ anhydrous), which acts as a seed to provide a surface for crystal growth. Sodium sulfate crystals in amounts of 0, 10, 40 and 60 g, respectively, were added to 100 mL of supersaturated brine in a beaker. The slurry was mixed for 10 minutes and the temperature was maintained at 40 ° C. Crystallization was not observed in the absence of sodium sulfate crystals. Immediately after mixing, a sample was taken and filtered to remove crystals. Samples were analyzed for sulfate. The sulfate concentrations for the different amounts of added sodium sulfate crystals were 1,680, 1,400, 1,370 and 1,350 meq / L, respectively. Thus, it can be concluded that the addition of seeds helps to reduce sodium sulfate supersaturation. Moreover, it was observed that primary nucleation does not occur without the addition of sodium sulfate seeds. It should be noted that the conditions for these experiments have not yet been optimized. Thus, for example, it is expected that an increase in mixing time may result in a further decrease in sulfate concentration.

この実験は、Belsperse（商標）164の存在が、硫酸ナトリウムの結晶化を有効に抑制することを示した。ブランク実験については、硫酸ナトリウムの一次核形成は、非常に細かい粒子の形成を生じた。あまり明白ではないが、この現象はまた、2.1ppmのBelsperse（商標）164を用いた実験についてもまた観察された。 Example 4
Brine with 275 g / L sodium chloride was produced. The brine was then saturated with sodium sulfate at 35 ° C. by adding excess sodium sulfate (ie, about 100 g / L Na ₂ SO ₄ ). The resulting solution was filtered using filter paper and a Buchner type filter to remove solid sodium sulfate. A sample was taken from this solution and the initial sulfate concentration was determined by ion chromatography. Four stirred glass beakers covered with lids were charged with 0.5 kg of the resulting clear solution. Dilute Belsperse ™ 164 solution was added to reach final Belsperse ™ 164 concentrations of 0, 2.1, 22 and 82 ppm, respectively. The temperature of the glass beaker was controlled by a heated jacket containing water. The initial temperature was set at 35 ° C. and the sulfate concentration was determined by ion chromatography. This is a blank experiment (entry 1 in Table 1). Thereafter, the temperature was gradually raised to 95 ° C. in all four beakers. The solubility of sodium sulfate decreases with increasing temperature. At 80 ° C., sodium sulfate was crystallized visually in the absence of Belsperse ™ 164. At 95 ° C., it was observed that sodium sulfate also crystallized in the presence of 2.1 ppm Belsperse ™ 164, whereas in other beakers with 22 and 82 ppm Belsperse ™ 164, crystallization occurred. Did not occur. At the end of the experiment, a sample was taken and filtered through a small 0.45 μm filter. Sulfate concentration was determined by ion chromatography. The results are summarized in Table 1:

This experiment showed that the presence of Belsperse ™ 164 effectively inhibited crystallization of sodium sulfate. For the blank experiment, primary nucleation of sodium sulfate resulted in the formation of very fine particles. Although less obvious, this phenomenon was also observed for experiments with 2.1 ppm Belsperse ™ 164.

Comparative Example 5
Another experiment was performed using a newly installed flat sheet of NF ™ 270 polyamide thin film NF membrane (from The Dow Chemical Company). Membrane sheets were tested simultaneously in a DSS lab stack unit, which was operated at an orthogonal flow rate of 600 L / hour. A total membrane surface area of 0.18 m ² was installed. The feed solution was prepared by dissolving Na ₂ SO ₄ in deionized water in an amount of 61.7 g per kg of total feed, and then sodium chloride (NaCl (71381), Fluka Chemie GmbH, CH- 9471 Buchs (from Switzerland) was added in an amount of 246 g / kg of total feed. No crystal growth inhibitor was added. The slurry feed was filtered through Whatman 54 (20-25 μm) filter paper to remove undissolved solids. The resulting clear filtrate was concentrated in a batch mode with a nanofiltration unit to a concentration factor CF = 1.08, which means that the permeate was discharged. On the other hand, the concentrate was returned to the feed container until the concentration rate, ie the ratio of initial feed weight to concentrate weight, was 1.08. Subsequent operation under this condition, with both concentrate and permeate recycled to the feed vessel, was maintained for 2 hours. Membrane filtration was performed at an operating pressure of 35 bar and a temperature of 35 ° C. The concentrate under these conditions contained 1,162 meq / L SO ₄ ^2- . The concentrate was then further concentrated to a concentration factor CF = 1.16 (relative to the first feed) by draining the permeate from the unit and recycling the concentrate to the feed vessel. During batchwise concentration up to CF = 1.16, crystallization was observed in the feed vessel and very small Na ₂ SO ₄ crystals were produced. The concentrate mother liquor at CF = 1.16 contained 1,221 meq / L SO ₄ ²⁻ . After 2 hours of operation at CF = 1.16, the mother liquor concentration dropped to 1,174 meq / L SO ₄ ²⁻ . This is because still more crystals were formed.

Comparative Example 6
Another experiment was performed using a newly installed flat sheet of NF ™ 270 polyamide thin film NF membrane (from The Dow Chemical Company). Membrane sheets were tested simultaneously in a DSS lab stack unit, which was operated at an orthogonal flow rate of 600 L / hour. A total membrane surface area of 0.18 m ² was installed. The feed solution was prepared by dissolving Na ₂ SO ₄ in deionized water in an amount of 62.6 g per kg of the total feed, sodium chloride (NaCl (71381), Fluka Chemie GmbH, CH-9471 Buchs (Buchs) (from Switzerland) was added in an amount of 235 g / kg of total feed. After dissolving Na ₂ SO ₄ and before dissolving NaCl, 2.4 mg of Belsperse ™ 164 crystal growth inhibitor was added per kg of total feed (as a 40% solution of 6.1 mg / kg). The slurry feed was filtered through Whatman 54 (20-25 μm) filter paper to remove undissolved solids. The resulting clear filtrate is concentrated batch-wise in a nanofiltration unit to a concentration factor CF = 1.08, which allows the permeate to drain until the ratio of initial feed weight to concentrate weight is 1.08. Meanwhile the concentrate has been returned to the feed container. Subsequent operation under this condition, with both concentrate and permeate recycled to the feed vessel, was maintained for 2 hours. The concentrate under these conditions contained 1,209 meq / L SO ₄ ^2- . The concentrate was then further concentrated to a concentration rate of CF = 1.16 (relative to the first feed) by draining the permeate from the unit and recycling the concentrate to the feed vessel. Membrane filtration was performed at an operating pressure of 35 bar and a temperature of 35 ° C. During batchwise concentration up to CF = 1.16, crystallization was observed in the feed vessel and very small Na ₂ SO ₄ crystals were produced. Immediately after reaching a concentration factor of CF = 1.16, the concentrate mother liquor contained in particular 1,265 meq / L SO ₄ ²⁻ . After running for 2 hours at CF = 1.16, the mother liquor concentration dropped to 1,208 meq / L SO ₄ ²⁻ , while still more crystals formed. Compared to Example 6, it was hardly possible to increase the maximum sulfate concentration at which crystallization was avoided.

Example 7
Another experiment was performed using a newly installed flat sheet of NF ™ 270 polyamide thin film NF membrane (from The Dow Chemical Company). Membrane sheets were tested simultaneously in a DSS lab stack unit, which was operated at an orthogonal flow rate of 600 L / hour. A total membrane surface area of 0.18 m ² was installed. The feed is made by dissolving Na ₂ SO ₄ in deionized water in an amount of 62.6 g / kg of the total feed, sodium chloride (NaCl (71381), Fluka Chemie GmbH, CH-9471 Buchs (Buchs) (from Switzerland) was added in an amount of 234 g / kg of total feed. After dissolving Na ₂ SO ₄ and before dissolving NaCl, 25.3 mg of Belsperse ™ 164 crystal growth inhibitor per kg of total feed was added. The slurry was filtered through Whatman 54 (20-25 μm) filter paper to remove undissolved solids. The resulting clear filtrate is concentrated batch-wise in a nanofiltration unit to a concentration factor CF = 1.09, which means that the permeate is discharged until the ratio of initial feed weight to concentrate weight is 1.09. Meanwhile the concentrate has been returned to the feed container. Subsequent operation under this condition, with both concentrate and permeate recycled to the feed vessel, was maintained for 2 hours. The concentrate under these conditions contained 1,190 meq / L SO ₄ ^2- . The concentrate was then further concentrated to a concentration factor of CF = 1.17 (relative to the first feed) by draining the permeate from the unit and recycling the concentrate to the feed vessel. Membrane filtration was performed at an operating pressure of 35 bar and a temperature of 35 ° C. No crystallization of Na ₂ SO ₄ was observed. Operation under this condition with both concentrate and permeate recirculation was maintained for 2 hours. The concentrate with CF = 1.17 contained in particular 1,287 meq / L SO ₄ ^2- and did not change during 2 hours of operation at CF = 1.17.

2 is a flowchart of the method of the present invention.

Claims (11)

A method for removing at least part of a soluble alkali metal or ammonium salt of a divalent anion from an aqueous salt solution containing an aqueous alkali metal or ammonium ion and a divalent anion, comprising the following steps:
-Having a sodium chloride concentration between 150 g / L and saturation in the presence or absence of a crystal growth inhibitor for sodium chloride, or above saturation in the presence of a crystal growth inhibitor for sodium chloride Obtaining an aqueous salt solution having a sodium chloride concentration of, wherein the aqueous salt solution may optionally comprise a crystal growth inhibitor for an alkali metal or ammonium salt of a divalent anion;
-Acidifying the solution to a pH below 11.5, if necessary, while maintaining a sodium chloride concentration of at least 150 g / L;
-If the concentration of the crystal growth inhibitor for the alkali metal or ammonium salt of the divalent anion in the salt aqueous solution is less than 20 mg / L, the resulting salt aqueous solution will suppress the crystal growth for the alkali metal or ammonium salt of the divalent anion. Adding the crystal growth inhibitor in an amount so as to contain at least 20 mg / L of the agent;
-The resulting solution is subjected to a membrane filtration step, whereby the aqueous salt solution is subsaturated for a salt water stream (concentrate) that is supersaturated for salts containing divalent anions and for salts containing divalent anions. Separating it into a salt water stream (permeate);
-If the concentration of the crystal growth inhibitor for alkali metal or ammonium salt of divalent anion in the concentrate is less than 20mg / L, the concentrate will suppress crystal growth for alkali metal or ammonium salt of divalent anion Adding the crystal growth inhibitor in an amount so as to contain at least 20 mg / L of the agent;
-Subjecting the resulting concentrate to a crystallization process;
-Removing the crystallized divalent anion alkali metal or ammonium salt; and
-Optionally comprising recirculating at least part of the mother liquor of the crystallizer to an aqueous salt solution for resubmission to the membrane filtration step. The process of claim 1 wherein the saline solution is saturated with sodium chloride or has a sodium chloride concentration between 275 g / L and saturation. The method according to claim 1 or 2, wherein the divalent anion is sulfate or carbonate. The method according to any one of claims 1 to 3, wherein the alkali metal is sodium. 5. A crystal growth inhibitor for an alkali metal or ammonium salt of a divalent anion is present in the concentrate in an amount of at least 25 mg / L (preferably 75 to 150 mg / L). The method described. Crystal growth inhibitors for alkali metal or ammonium salts of divalent anions include humic acid, polymaleic acid, polyacrylic acid, sugars, oligopeptides, polypeptides, and two or more carboxylic acid groups or carboxyalkyl groups, and 6. A process according to any one of the preceding claims, optionally selected from the group consisting of polymers further having phosphate, phosphonate, phosphino, sulfate and / or sulfonate groups. Step of acidification, using at least CO _2, any one method according to claims 1-6 carried out in a scrubber. The method according to any one of claims 1 to 7, wherein the pH of the aqueous salt solution to be subjected to the membrane filtration step is 2 to 11.5. The method according to any one of claims 1 to 8, wherein the membrane filtration step is performed by nanofiltration. The method according to any one of claims 1 to 9, wherein the crystallization step is carried out by adding a seed of an alkali metal or ammonium salt of a divalent anion to be crystallized. The method according to claim 1, wherein the permeate is used as a raw material in sodium chloride production or soda production.

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